Separation of the boundary layer from an airfoil can cause a sharp drop in the lift generated by the airfoil. This can have such adverse results as reduced performance or an aircraft stalling and falling from the sky.
As is apparent from the following U.S. patents, for example, major efforts have been devoted to solutions of this separation problem: U.S. Pat. No. 3,085,740 issued Apr. 6, 1963 to Wagner for END INLET JET PUMP FOR BOUNDARY LAYER CONTROL SYSTEM; U.S. Pat. No. 3,161,377 issued Dec. 15, 1964 to Balluff for APPARATUS FOR CONTROLLING AIRCRAFT; U.S. Pat. No. 3,604,661 issued Sep. 14, 1971 to Mayer, Jr. for BOUNDARY LAYER CONTROL MEANS; and U.S. Pat. No. 3,693,913 issued Sep. 26, 1972 to Barland et al. for FLUID FLOW CONTROL DEVICE.
One of the significant problems which the foregoing patentees and other investigators have not successfully addressed, however, is the control of boundary layer separation at the intersection between an airfoil and an adjoining structure; for example, at the intersection between the wing and the body of an aircraft. Control of the boundary layer on an airfoil at an intersection with an adjoining structure has been experimentally attempted in two dimensional wind tunnel tests in which the wing or airfoil being evaluated spans the wind tunnel and the tunnel wall boundary layer as a consequence causes premature stall; i.e., separation of the boundary layer from the airfoil in the vicinity of the tunnel wall. One approach to solving this boundary layer separation problem used slot-type nozzles to direct a sheetlike jet of fluid along the wind tunnel wall and over a turntable on which the airfoil was supported. A second scheme used areawide suction in this same region of the test set-up.
Neither scheme was particularly successful. The more severe, separation-promoting pressure gradient exists along the surface of the airfoil; and this surface is left unprotected by the flow induced via the positive or negative pressure generated by the boundary layer control scheme. As a consequence, flow separation continued to occur at only a small distance from the tunnel wall and the turntable.
Furthermore, the amount of fluid employed by the pressurized sheet of fluid might well make that approach impractical for jet aircraft and similar applications in which the compressed air available for uses other than in propulsion units is very limited.
Thus, boundary layer separation continues as a problem in circumstances involving the juncture of an airfoil and an adjoining structure despite the efforts of previous investigators.